In order to provide large bandwidth and high-capacity coverage, Access Nodes (ANs) in a network typically share the entire spectrum available in the network. For example, in a millimeter wave (mmW) network, a number of “high-capacity coverage islands” are deployed and the entire spectrum available in the mmW network is reused by each of these islands.
FIG. 1 shows an exemplary scenario of spectrum reuse. Two networks, a network 110 and a network 120, are shown in FIG. 1. The coverage area of the network 110 and the coverage area of the network 120 partially overlap each other, as shown by the dashed lines in FIG. 1. The networks 110 and 120 may belong to different network operators and may be allocated with the same spectrum. The network 110 includes two ANs, AN 111 and AN 112, and the network 120 includes two ANs, AN 121 and AN 122. All the ANs 111, 112, 121 and 122 in FIG. 1 share the same spectrum. FIG. 1 also shows three User Equipments (UEs) 101, 102 and 103. Here, the UE 101 is served by the AN 111, the UE 102 is served by the AN 112 and the UE 103 is served by the AN 122. It can also be seen from FIG. 1 that the UE 102 is located in the overlapped area and it is assumed here that the UE 102 is within the coverage of each of the ANs 111, 112 and 121. In the following, operations performed by the UE 102 will be described as an example, without loss of generality.
The UE 102 may need to obtain information related to a neighboring AN (if any), e.g., to discover a neighboring AN or to perform measurements related to a neighboring AN. Such neighbor discovery or measurement can be periodical or event-triggered. For example, the UE 102 may need to measure a Reference Signal Received Power (RSRP) from an intra-network AN (i.e., an AN of its serving network 110), if any. In doing so, it first needs to listen to a beacon channel to detect if there is any beacon broadcast by a neighboring AN.
A beacon broadcast by a particular AN contains a sync signal sequence (such as Primary Synchronization Signal (PSS) or Secondary Synchronization Signal (SSS) in Long Term Evolution (LTE) system), a reference signal (RS) sequence and system information. The system information includes a Network Identity (NI) which globally uniquely identifies the network the AN belongs to. In this context, a “network” can be a Public Land Mobile Network (PLMN) (in this case the NI can be a PLMN ID) and different “networks” are typically managed by different network operators. The system information further includes an AN Identity (AI) which uniquely identifies the AN locally within the network it belongs to. An example of the AI is the evolved NodeB (eNB) ID in LTE. The system information further includes a Physical AN Identity (PANI) associated with physical layer functions of the AN. A PANI is uniquely associated with a combination of sync signal sequence and reference signal sequence (and their time and frequency locations) and such association is common among different networks. An example of the PANI is the Physical Cell ID (PCI) in LTE.
If there is a beacon broadcast by the AN 121 on the beacon channel, the UE 102 first detects the sync signal sequence blindly. When the UE 102 successfully detects the sync signal sequence, it knows the PANI from the detected sync signal sequence and thus determines the reference signal sequence in the beacon based on the PANI. At that time, the UE 102 measures the received power of the reference signal sequence as the RSRP. Then, the UE 102 derives a channel estimation based on the determined reference signal sequence, and finally decodes the system information in the beacon based on the channel estimation. From the NI included in the system information, the UE 102 knows that the beacon is broadcast by the AN 121 which is an inter-network AN (i.e., an AN of a network different from its serving network 110) instead of an intra-network AN. Thus, the measured RSRP from the AN 121 will be discarded, without being reported to the AN 112.
Similarly, if there is a beacon broadcast by the AN 111 on the beacon channel, the UE 102 also needs to detect the sync signal sequence blindly, determine the reference signal sequence, measure the RSRP and decode the system information. From the NI included in the system information, the UE 102 knows that the beacon is broadcast by the AN 111 which is an intra-network AN. Then, the measured RSRP from the AN 111 will be reported to the AN 112.
That is, the UE cannot identify whether a beacon it detects originates from an intra-network AN or an inter-network AN until it successfully decodes the system information. In the intra-network measurement as discussed above, the RSRP from the inter-network AN is also measured, which is unnecessary and inefficient. This causes a certain amount of delay in the measurement-report process, which may be intolerable in some circumstances. For example, in order to support UE mobility in LTE, the UE is required in an event-triggered measurement to send a measurement report within a short period (e.g., 200 ms for intra-frequency non-DRX (Discontinuous Reception) scenario). The UE may fail to meet this requirement due to the delay.
Even worse, there is a case where the UE 102 may fail to decode the system information. There are typically a limited number of PANIs available (i.e., 504 PCIs in LTE) and thus different ANs in the same or different network(s) may have the same PANI. With cell planning, neighboring ANs of the same network can be allocated with different PANIs by the network operator. However, neighboring ANs from different networks (e.g., AN 111 and AN 121) may have the same PANI due to lack of inter-operator coordination. In this case, when the UE 102 receives two beacons simultaneously, one from the AN 111 and the other from the AN 121 (it is assumed here that the AN 111 and the AN 121 are synchronized and thus the beacons are aligned with each other), the beacons from the AN 111 and the AN 121 contain the same sync signal sequence and the same reference signal sequence. The UE 102 cannot realize that it is receiving beacons from different ANs. Hence, it will successfully detect the sync signal sequence and accordingly measure a combined RSRP (which is meaningless) and derive a combined channel estimation of a channel between the UE 102 and the AN 111 and a channel between the UE 102 and the AN 121. With such combined channel estimation, the UE 102 cannot decode the system information in either of the beacons. In this case, it is impossible for the UE 102 to perform intra-network or inter-network measurement.
There is thus a need for an improved solution for identifying an intra-network AN or an inter-network AN.